how can I be sure these counts accumulated since reboot of the server? Without the knowledge if/when the counters were reset the last time, the output of vxstat is of limited use. Instead, I have to reset the counters myself, thereby losing potentially valuable data that has accumulated over a long time.

So is there a way to determine the time of the last reset?

This example is from a very old VxVM 4.1 on Solaris 10, but the vxstat man page from 5.1 has no info about the time of the last reset either.